Standardization of the LTE (Long Term Evolution) system (Rel. 8 and Rel. 9), which is a wireless communication system for 3.9th generation mobile phones, has been completed, and the LTE-A (LTE-Advanced, also referred to as IMT-A, for example) system (after Rel. 10) as a more advanced version of the LTE system is now being standardized as one of 4th generation wireless communication systems.
In Rel. 12 of the LTE-A system, scenario of arranging a pico base station apparatus (also referred to as Pico eNB; evolved Node B, Small Cell, Low Power Node, or Remote Radio Head) having a small cell coverage has been studied. A terminal apparatus (user apparatus, UE, mobile station apparatus) connected to the pico base station apparatus is considered to be simultaneously connected to a macro base station apparatus having a large cell coverage or connected only to the pico base station apparatus.
It has been studied that, when pico base station apparatus are arranged densely, a state where a part of signals transmitted by a pico base station apparatus at all times is not sent (also referred to as an Off-state or a Dormant mode) is provided according to a traffic amount or the like from a viewpoint of a problem of interference imparted to other pico base station apparatuses and power consumption (refer to NPL 1). In the pico base station apparatus, by switching a state where data of the downlink is transferred (On-state) and the Off-state according to the traffic amount, it is possible to reduce influence of interference and increase system throughput. On the other hand, when a transition time between the On-state and the Off-state of the pico base station apparatus is long, it takes a long time to start communication of the terminal apparatus, so that there arises a problem that overhead in which the terminal apparatus is connected to the pico base station apparatus under an environment where the traffic amount changes drastically becomes large and an effect of improving the system throughput is reduced. Thus, a method for shortening time required until transmission and reception of data is allowed after the terminal apparatus is connected to the pico base station apparatus in the Off-state has been studied.
In a case where the terminal apparatus sets a serving cell (or CC: Component Carrier) formed by the macro base station apparatus as a primary cell, in order to perform connection with the pico base station apparatus, it is necessary to add the CC formed by the pico base station apparatus as a secondary cell or to perform handover to the CC formed by the pico base station apparatus. In this case, RRM (Radio Resource Management) measurement accounts for a large proportion of the overhead needed for the terminal apparatus to perform handover to the CC formed by the pico base station apparatus. Thus, it has been proposed to realize reduction in the overhead by enabling the RRM measurement also for the pico base station apparatus in the Off-state by the terminal apparatus, and one method thereof is to transmit a Discovery Signal (DS, also referred to as a DRS: Discovery Reference Signal) by the pico base station apparatus in the Off-state (refer to NPL 2). As a method for transmitting the discovery signal, it has been studied to use a part of REs (resource elements) used by a CRS (Cell-Specific Reference Signal), a CSI-RS (Channel State Information Reference Signal), and a PSS/SSS (Primary Synchronization Signal/Secondary Synchronization Signal), and it has been studied that the pico base station apparatus transmits the discovery signal in both of the Off-state and On-state.